U.S. Republican presidential candidate Donald Trump lashed out Tuesday at the party's top elected official and other Republicans for abandoning his campaign, while acknowledging that it is "hard to do well" when they are giving him "zero support."

Trump, in a series of postings on his Twitter account, attacked House Speaker Paul Ryan as a "very weak and ineffective leader" after Ryan told other Republicans on Monday that he would no longer campaign for Trump or defend his offensive comments, which have turned voters against the brash real estate mogul.

The emergence of a 2005 tape in which Trump is heard making crude comments about women has led Ryan and some other Republicans to spurn his candidacy. In response, Trump tweeted Tuesday, "It is so nice that the shackles have been taken off me, and I can now fight for America the way I want to." He said Democrats "have always proven to be far more loyal to each other than the Republicans!"

Ryan, while still endorsing Trump, all but conceded that Democrat Hillary Clinton would defeat Trump in the November 8 election. Ryan said he would campaign for Republican colleagues to try to maintain the Republican House majority, rather than see a Democratic takeover that would give Clinton a "blank check" for legislative action.

Some conservative House members supporting Trump protested Ryan's abandonment of the Republican standard-bearer, saying there was no reason to quit on him four weeks from Election Day.

Numerous Republican elected officials, including the party's 2008 presidential candidate, Arizona Senator John McCain, have retracted their support for Trump in recent days or implored him to drop out of the race after the 2005 tape surfaced last week in which Trump bragged that he could grope women with impunity because he was a celebrity. Trump said there was "zero chance" he would quit.

FILE - Paul Ryan.

Trump, making his first run for elected office, has apologized for the lewd remarks made to a television personality, caught by a live microphone, while riding on a bus on a Hollywood back lot as he prepared to make a cameo appearance on a television soap opera. At Sunday's debate, he described them as "locker room talk."

Trump was losing ground to Clinton, who is looking to become the country's first female president, before the existence of the tape was disclosed by The Washington Post. An NBC News/Wall Street Journal poll conducted after the tape was unearthed showed her surging to a 46 percent-to-35 percent advantage. Other polling that encompassed the days before and after the tape was aired widely on U.S. news shows also showed Clinton gaining ground nationally and in battleground election states where the race will be determined.

Trump acknowledged his uphill fight in a campaign appearance late Monday in the eastern state of Pennsylvania, one of the key states where he is trailing.

"I may be limping across that finish line, but we're going to get across," Trump told supporters.

He vowed to make six campaign stops a day in the last week of the campaign to try to overtake Clinton, the wife of former President Bill Clinton.

Trump said that if more damaging tapes emerged from the many years during which he regularly discussed his sexual exploits on radio shows, then he would continue "to talk about Bill and Hillary doing inappropriate things."

At Sunday's debate, he said Hillary Clinton "viciously" disparaged women who alleged that her husband had made unwanted sexual advances on them in a series of encounters two decades ago. Trump positioned three of the women in front-row seats to watch the debate.

Clinton, campaigning in the large midwestern state of Ohio, mocked Trump's onetime career as the host of a television reality show while she served as the country's top diplomat from 2009 to 2013 and his claim that she has nothing to show for 30 years in American public life.

"On the day that I was in the Situation Room watching the raid that brought Osama bin Laden to justice, he was hosting 'Celebrity Apprentice,' " she said. "So if you want to talk about what we've been doing the last 30 years: Bring. It. On."